$('.js-agree-radio').change(function () {
  if ($(this).prop('checked')) {
    $('.js-upgrade-btn').removeClass('disabled');
  } else {
    $('.js-upgrade-btn').addClass('disabled');
  }
});

$('.js-upgrade-btn').click(function () {
  $.post($(this).data('url'), function (html) {
    $('.modal').html(html);
  });
});